The present invention relates to cooling roll, a method for manufacturing a magnet material, a ribbon shaped magnet material, a magnetic powder and a bonded magnet.
A bonded magnet prepared by bonding a magnetic powder with a bonding resin is used for motors and various actuators by taking advantage of its wide degree of freedom of configuration.
Magnet materials constituting the bonded magnet described above are manufactured by, for example, an quenching method using a quenching type ribbon manufacturing apparatus. The manufacturing method is called as a single roll method when the quenching type ribbon manufacturing apparatus comprises a single cooling roll.
In the single roll method, a thin foil (ribbon) shaped magnet material, or a quenched ribbon is continuously manufactured by the steps comprising heating and melting a magnet material with a prescribed alloy composition, ejecting the molten liquid from a nozzle to allow it to collide with the circumference face of a rotating cooling roll, and quenching and solidifying the molten liquid by allowing it to contact the circumference face. The quenched ribbon is pulverized into a magnetic powder, and a bonded magnet is manufactured using this magnetic powder.
A roll (without no surface coating) made of a metal or an alloy, which has high heat conductivity is used for the cooling roll. Alternatively, a surface layer plated with Cr having a lower heat conductivity than the roll base is provided on the surface of the roll for the purpose of improving durability of the roll.
However, when the roll having no surface coating layer as described in the former case above is used, the magnet material is apt to form an amorphous phase due to very rapid cooling rate on the roll contact surface (the surface in contact with the circumference of the cooling roll) of the quenched ribbon obtained. On the free surface (the face opposed to the roll contact surface), on the contrary, the crystal grain size is coarsened due to slow cooling speed as compared with the roll contact surface, resulting in deterioration of magnetic properties.
While heterogeneous distribution of the crystal grain size as described in the latter case is a little relaxed by providing the surface layer comprising a Cr plating layer having a lower heat conductivity as compared with the roll base, the method involved the following problems.
When the Cr plating layer is formed by electroplating on the base, the growth rate of the Cr plating layer usually shows a significant difference depending on the plating site due to surface roughness of the base, and the surface roughness of the base is remarkably reflected on the surface roughness of the plating layer. Accordingly, large voids are formed between the plating layer and the quenched ribbon due to the large surface roughness when the plating layer obtained as described above is directly utilized as the surface layer, causing a large difference in the cooling rate at different sites on the surface layer. As a result, the crystal grain size distribution in the quenched ribbon turns out to be heterogeneous to make it impossible to obtain stable magnetic properties.
Accordingly, a machining such as surface grinding or polishing is usually applied for smoothing the surface after plating. In the machining step applied on the rotating cooling roll, however, uniform processing of the surface along the circumference direction is impossible due to eccentric rotation and mechanical shift and vibration of the cooling roll when the machining as described above is applied to the cooling roll, finally causing heterogeneous distribution of the thickness of the Cr plating layer obtained.
Heat conduction characteristics of the quenched ribbon obtained largely differ among the different sites on the plating layer when the thickness of the plating layer is heterogeneous. Consequently, the alloy of the quenched ribbon has a heterogeneous distribution of the crystal grain size to unable stable and high magnetic properties to be obtained.
The object of the present invention is to provide a cooling roll and a method for manufacturing a magnet material that is able to provide a highly reliable magnet having good magnetic properties, and a ribbon shaped magnet material, magnetic powder and bonded magnet.
The object of the present invention can be attained by the following aspects and related features described in (1) to (24) below.
(1) The first cooling roll according to the present invention for manufacturing a magnet material has a surface layer on an entire outer circumference of a roll base of the cooling roll, wherein the maximum thickness Tmax and the minimum thickness Tmin of the surface layer satisfy the relation of 1.01xe2x89xa6Tmax/Tminxe2x89xa63.
(2) The second cooling roll for manufacturing a magnet material has a roll base and a surface layer provided on an entire outer circumference thereof, wherein the surface roughness Ra of a bonding face between the roll base and the surface layer is 0.03 to 8 xcexcm.
(3)Preferably, the surface layer is manufactured without applying any machining on its surface.
(4) Preferably, the surface layer is formed by a chemical vapor deposition (CVD) method or a physical vapor deposition (PVD) method.
(5) Preferably, the surface layer comprises a ceramic.
(6) Preferably, the mean thickness of the surface layer is 0.5 to 50 xcexcm.
(7) Preferably, the surface roughness Ra of the surface layer is 0.03 to 8 xcexcm.
(8) Preferably, the radius of the cooling roll is 50 to 1000 mm.
(9) Preferably, the magnet material is an alloy comprising rare earth elements, transition metals and boron.
(10) The first method for manufacturing the magnet material according to the present invention comprises manufacturing a ribbon shaped magnet material by a quenching method using the cooling roll described in (1) or (2).
(11) The second method for manufacturing the magnet material according to the present invention comprises manufacturing a ribbon of a magnet material by ejecting a molten liquid of the magnet material from a nozzle in an atmospheric gas, and allowing the molten liquid to collide with the circumference face of the cooling roll according to (1) or (2) rotating relative to the nozzle, followed by cooling and solidifying the molten liquid.
(12) Preferably, the atmospheric gas is an inert gas.
(13) Preferably, the circumferencial speed of the cooling roll is 5 to 60 m/sec.
(14) Preferably, the mean thickness of the ribbon shaped magnet material obtained is 10 to 50 xcexcm.
(15) Preferably, the ribbon shaped magnet material obtained comprises a composite microstructure in which soft magnetic phases and hard magnetic phases are distributed in adjoining relation to one another.
(16) The ribbon shaped magnet material according to the present invention is manufactured by the method according to any one of (10) to (12).
(17) The magnetic powder according to the present invention is obtained by pulverizing the ribbon shaped magnet material manufactured by the method according to any one of (10) to (12).
(18) Preferably, the magnetic powder described above is subjected to at least one time of heat treatment during the manufacturing process or after manufacturing.
(19) Preferably, the magnetic powder comprises a single phase microstructure or a composite phase microstructure with a mean crystal grain size of 500 nm or less.
(20) Preferably, the magnetic powder has a mean grain size of 0.5 to 150 xcexcm.
(21) The bonded magnet according to the present invention is prepared by bonding the magnetic powder according to (17) or (20) with a bonding material.
(22) Preferably, the bonded magnet contains 75 to 99.5% of the magnetic powder.
(23) Preferably, the bonded magnet has a coercive force HcJ of 320 to 900 kA/m.
(24) Preferably, the bonded magnet has a maximum magnetic energy product (BH)max of 60 kJ/m3 or more.